The present invention relates to interwoven spreading codes such as Barker codes which are used in pulse compression or pulse encoding, such as may be used in direct sequence spread spectrum communications. Although the present invention is discussed primarily in the context of direct sequence spread spectrum signals, the present invention is not to be limited to this specific context as Barker codes and other spreading codes can be used in other contexts, especially in applications relating to high noise low signal environments.
In a spread spectrum system a signal occupies a higher bandwidth than the minimum necessary for transmission of information. The baseband is spread through using a code that is independent of the data to be sent. Direct sequence is one type of technique where the data signal is multiplied by a code signal. The code can be a Barker code. At the receiver, the original data signal is recovered by correlation of the received signal with a synchronized replica of the coding signal used to spread the baseband. Thus Barker codes can be used for spreading.
Barker codes can be defined as a sequence of length N for which the aperiodic autocorrelation function is less than or equal to 1/N away from 0. Barker codes can exist for binary or non-binary codes. Binary Barker codes have been found for lengths of 2, 3, 4, 5, 7, 11, and 13. A Barker code is a series of digits ai=±1 of length N≧2 such that
                        ∑                  i          =          1                          N          -          k                    ⁢                        a          i                ⁢                  a                      i            +            k                                    ≤      1    ⁢                  ⁢    for    ⁢                  ⁢    all    ⁢                  ⁢    1    ≤  k  <  N
Barker codes are used in pulse compression, or pulse coding. Barker codes can be used to compare two signals resulting in a maximum output if the two signals match and either a zero or a constant minimum value in other cases. This comparison process is generally called correlation. One bit of each input line is examined at a time, these bits are multiplied and the individual results are added. Barker codes are not the only type of code that is used for spreading, as other spreading codes are used.
Despite these advantages, problems remain. One of the most significant problems with using Barker codes is the uneven envelope of the pulses when a limited bandwidth is required. This uneven envelope allows less energy per pulse to be transmitted, thus reducing the sensitivity of the receiver.
Therefore, it is a primary object, feature, or advantage of the present invention to improve upon the state of the art.
Another object, feature, or advantage of the present invention is to provide a code that provides for increased uniformity in amplitude and thus a more even envelope when a limited bandwidth is required.
Yet another object, feature, or advantage of the present invention is to provide a code that provides for increased energy per pulse to be generated.
A further object, feature, or advantage of the present invention is to provide a code that provides substantially the same autocorrelation properties as those associated with a Barker code.
One or more of these and/or other objects, features, or advantages of the present invention will become apparent from the specification and claims that follow.